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Chemical Formula

Contents

Key Stage 3

Meaning

A Chemical Formula is a simple way to show the number and type of atoms in a chemical.

Singular Noun: Chemical Formula
Plural Noun: Chemical Formulae

About Chemical Formulae

Chemical Formulae show which elements are in a molecule.
The number of each element in a molecule is shown with a number written just after and lower than the element.

Examples

H2O CO2 CH4 C2H5OH
Water has 2 Hydrogen atoms and 1 Oxygen atom. Carbon Dioxide has 1 Carbon atom and 2 Oxygen atoms. Methane has 1 Carbon atom and 4 Hydrogen atoms. Ethanol has 2 Carbon atoms, 6 Hydrogen atoms and 1 Oxygen atom.

Key Stage 4

Meaning

A Chemical Formula is a way to show the numbers of atoms of each element in a chemical.

About Chemical Formulae

Chemical Formulae show which elements are in a molecule.
The number of each element in a molecule is shown with subscript after that element.
Some chemical formulae have brackets which indicate that the elements in the bracket are multiplied by the number after the bracket.

Some chemical formulae are written with the elements at specific points, not all grouped together, to show where different elements are bonded. This is called a structural formula.

Examples

HCl H2SO4 C3H8O C3H8O
Hydrochloric Acid molecules have 1 Hydrogen atom and 1 Chlorine atom. Sulphuric Acid molecules have 2 Hydrogen atoms, 1 Sulphur atom and 4 Oxygen atoms. Propanol or Propan-1-ol has 3 Carbon atoms and 8 Hydrogen atoms and 1 Oxygen atom. This has the same chemical formula as Propan-2-ol but a different shape. This may be written with the structural formula CH3CH2CH2OH to show where the OH group is attached. Isopropyl Alcohol or Propan-2-ol has 3 Carbon atoms and 8 Hydrogen atoms and 1 Oxygen atom. This has the same chemical formula as Propan-1-ol but a different shape. This may be written with the structural formula CH3CHOHCH3 to show where the OH group is attached.

OCR

Chemical formulae, pages 36-37, Gateway GCSE Chemistry, Oxford, OCR